1. Field of the Invention
The present invention relates to a process for mobile reception of broadcast signals and a circuit for executing the process. More particularly, the present invention relates to a process and circuit for mobile reception of television.
2. Description of the Related Technology
Formation of an output signal which is supplied to the receiver by summing of input signals weighted according to quality criteria is already known from documents U.S. Pat. No. 5,528,581, DE 38 33 709 A1, DE 197 08 996 A1 and U.S. Pat. No. 5,465,271.
Compared to other methods of multipath reception, as are known from DE 39 26 336 A1, DE 196 36 125 A1 and DE 197 39 898 A1, in which at any time only the use of one (the best) individual signal is enabled and thus the output signal supplied to the receiver is at most as good as the best input signal, an improvement can be achieved compared to the best input signal by addition of weighted input signals.
This advantage however can only be achieved when the transit time differences of the received television (signals generally caused by different propagation paths) are determined and equalized before their addition. Without this compensation, as is the case in the diversity transmission system described in U.S. Pat. No. 5,528,581, the acquired useful information is not improved, but degraded.
Generally a path difference and thus a transit time difference of the received signals occur on different propagation paths of a signal. By summing of reception signals the received useful information is not improved, but degraded. It is therefore necessary to determine and equalize the transit time differences before addition.
Transit time equalization is not disclosed in U.S. Pat. No. 5,528,581.
In the diversity process of DE 38 33 709 A1, first the respective phase of the HF carrier oscillations of the various reception signals is determined and then phase correction is done for compensation of transit time differences.
Instead of the phases of the HF signals, also those of the pertinent IF signals can be determined and thus delay correction circuits can be controlled, as is the case in the diversity process according to DE 197 08 996 A1.
The phase synchronization however has the disadvantage which is serious for practical use that the determined phase differences of the HF(IF) signals are always in the range between 0 and 2π. A phase shift of 2π corresponds to a time shift by the period duration T of the corresponding carrier oscillation. If between the reception signals time shifts larger than the period duration T occur, they are no longer correctly equalized and addition does not lead to an improvement in quality. This process, also called “PCD” (phase controlled diversity), would only allow equalization of alternate routes up to 0.35 m for example at a receiving frequency of 855 MHz. When PCD is used in current television, for very small path differences non-pertinent image contents would therefore be added and thus cause incorrect video reproduction.
Synchronization of the individual reception signals to one another is possible correctly only when the transmitted useful information itself is used to control synchronization. In U.S. Pat. No. 5,465,271, a digital communications system is described in which digital message bits are transmitted. Here the digital message is organized in frames and slots which contain a preamble. This is a special bit sequence which is known to the receiver. In this way the receiver can compare the received data to the data sequence known to it beforehand and can shift the individual reception signals to one another accordingly.
This known type of synchronization cannot be used in transmission of analog television signals since they have no frames, slots and preambles. Otherwise in this communications system the “preambles” of the input signals can be decoded to a sufficient degree and evaluated in the receiver only when the input signals are undisturbed and have little noise. For weak input signals however this is not the case so that this type of synchronization is not suited for a process of the initially mentioned type.